Positron lifetime τ and momentum distribution (curve shape parameter S) measurements are performed to study the decomposition in an Al5.9 at% Ag alloy during isochronal and isothermal heat treatment of water‐quenched samples. The positron studies are supplemented by small‐angle X‐ray scattering experiments. It is proved that positrons become trapped by G.P. zones being free of any vacancy‐type or dislocation‐type defect and by semicoherent γ′‐precipitates. Positron trapping rates show the coarsening of G.P. zones simultaneously with the formation and growth of γ′ during isothermal aging above 170 °C. From the temperature dependence of τ and S the transformation of η‐zones into τ‐zones is concluded. The Ag content of zones changes from about 60 at% at 120 °C (η‐zones) to 35 at% at 190 °C (η‐zones) which is in good agreement with SAXS and AP‐FIM experiments. Evidence of a reversible change of the G.P. zone composition when alternating the temperature between 190 and 120 °C is found in both positron and SAXS experiments.

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The influence of small Mg additions (xMg ≦ 0.20 at%) on the upper Limit temperature T GPZdiss of the existence of Guinier‐Preston (GP) zones is determined by SAXS investigations. In order to avoid falsifications by the size dependence of T GPZdiss, in all cases, the reversion treatments start with GP zones having a mean size of RK = (2.1 ± 0.2) nm. The main results are: (i) up to xMg = 0.10 at% T GPZdiss = (118 ± 3)°C, i.e. only few Mg atoms are inserted into the GP zones, but if adding 0.20 at% Mg to the Al4.5 at% Zn mother alloy T GPZdiss is raised to (152 ± 3)°C; (ii) the solubility of Mg in the matrix of the Al4.5 at% Zn mother alloy is less than 0.20 at%.